text:
Can a flight instructor applicant take CFII as an inital rating and be endorsed for 61.185(a)(2) and (3)?
I have some questions about CFII endorsement.

Is it possible to do CFII as the initial flight instructor rating?

If an applicant wants to do CFII initial, could the applicant be trained and be endorsed for both 61.185(a)(2) and 61.185(a)(3)?

@757toga Not 100% sure, but I think OP is asking if CFII can be done as the initial flight instructor rating instead of the typical CFI.

@757toga I think the verbiage there would be better stated as "Could the applicant be trained and endorsed for 61.185(a)(2)&(3)?". I'll edit the question to that and see if that was what OP was trying to ask.

I’m a bit confused about what you are asking. If you’re asking if you can become a CFII without doing all the other CFI details, the answer is no. In practice, you become a CFI first, then a CFII secondly - even though sometimes the training seems to become concurrent.
As a CFII/MEI, I’ve actually had an initial flight instructor candidate whose goal was to become a CFII. As I remember (many years ago), there was absolutely no confusion about her training plan in my mind. I gave her the requisite flight and ground training, intertwining the CFI and CFII curriculums, scheduled her with a Flight Examiner, and she became a CFII. And there was absolutely no pushback on the Examiners’s end, other than a phone call from him from telling me she was well prepared, and impressed by her decision to go straight to CFII.
A good starting point for reference would be 14 CFR 61 Subpart H. It lists all applicable flight and ground requirements for becoming a flight instructor:
https://www.law.cornell.edu/cfr/text/14/part-61/subpart-H

Welcome to Av.SE, and thanks for sharing your experience with an instructor candidate taking the less common path.

Yes, CFII can be done as the initial flight instructor rating. However, the endorsement for that would be 61.185(a)(1) [FOI] and (3) [Instrument], not (2) [Recreational, private, and commercial certificate training]. 61.185(a)(2) would be done when the CFI rating training is done which actually covers the relevant training.
The best source I have for this being possible besides from personal experience is the PTS (Practical Test Standards) for Flight Instructor Airplane on page 21 on the table that lists "Flight Instructor Certificate and Rating(s) Held". Both IA (Instrument Airplane) and IH (Instrument Helicopter) are listed. This would presume that it is in fact possible to acquire these ratings prior to applying for the CFI certificate.

@757toga No, all three regs do not have to be accomplished for an initial CFI certificate (2) or rating (3). The regs themselves say " certificate/rating applicable to the (aircraft [2]) category". What it is saying is that if you are seeking instructor privileges for rec, private, or commercial, you use (a)(2). If you are seeking instructor privileges for instrument rating, then use (a)(3). SO a CFI initial would need (1) and (2), while a CFII initial would need (1) and (3).

I also have had experience with this situation. When I first started looking around at places to do my CFI at, I asked if it was possible to do CFII initial and they all answered yes. I would still need to go for my CFI in order to be able to do any sort of real teaching though. The main benefit of it is that it splits the FOI portion into the CFII checkride which shortens the CFI checkride. Not all the places would do it, but they said it was an option that had been done before. I didn't go that route though and went for standard CFI initial.


text:
When were the last subsonic turbojet-powered airliners retired from commercial passenger service?
The very first jetliners (Comet, Tu-104, Caravelle I-VI, 707-120/-220/-320, DC-8 series 10-30, original 720, CV-880) all used turbojet engines, turbofans not being available until later; when turbofans became a viable option, jetliner manufacturers switched over to turbofans en masse for their new designs (including the later versions of the Caravelle, 707/720, and DC-8), and some of the older, turbojet-powered airliners were converted to also use turbofans, but many already-built jetliners continued to fly under turbojet power for decades.
I have been unable to ascertain precisely how long subsonic1 turbojet-powered airliners continued to carry paying passengers in regular service.  Wikipedia says that the Tu-104 was withdrawn from passenger service in March 1979, and the Comet last carried paying passengers in 1981, but does not provide a citation for either statement, and I have been unable to find any information as to when the other turbojetliners were retired from passenger service.
Can someone help with this?

Yes, I am aware that the Concorde used turbojets; it does not count for the purposes of this question, as it was a supersonic aircraft, cruising at Mach 2, where turbojets are actually more efficient than turbofans, whereas this question concerns those early subsonic aircraft that were only built with turbojets because turbofans were not yet an option.

EDIT: The original intent of this question was to ask when the last subsonic turbojetliners in scheduled passenger service anywhere in the world were retired, but John K's comment raises the good point that these aircraft would have hung on in the dusty corners of the globe for a good long while after they were retired in places like the U.S. or western Europe.  Therefore:
When were paying passengers last flown in scheduled service on subsonic turbojet-powered airliners...

...in First and Second World countries (U.S., Canada, Europe, East Asia, Australasia)?
...anywhere in the world?

EDIT 2: The Trident 3B also does not count, as it was mostly turbofan-powered, using the turbojet only as an additional booster on takeoff for hot-and-high operations.

You'd have to define locality, because most of the obsolete stuff retired in North America would end up in South America for another couple of decades and you'd have seen old turbojet DC8s and such down there hauling freight probably into the 90s.

@JohnK: Good point.  Edited.

Perhaps related

As far as the T-104, Aeroflot retired the Tu-104 from civil service in March 1979 however it wasn't until after the Pushkin crash in February 7th 1981 that killed 52 people that the type was permanently removed from service.

Follow-up / safety actions
The remaining military Tu-104 aircraft were grounded following this accident.

Source: https://aviation-safety.net/database/record.php?id=19810207-0
The last commercial passenger flight of the Comet 4 was by Dan Air on November 9th 1980.

Comet 4s flew with BOAC until 1965 and with British airline Dan Air until 1980.

Source: http://www.bbc.com/future/story/20170404-the-british-airliner-that-changed-the-world
To answer the actual question of the last in service, that appears to be the B707 for all countries.
Trans World Airlines flew the last scheduled 707 flight for passengers by a US carrier on October 30, 1983

An era in U.S. aviation ended at 3:25 p.m. on Oct. 30, when TWA Flight 4, the last Boeing 707 to fly for a scheduled airline in the continental U.S., touched down at New York City's Kennedy International Airport.

Source: http://content.time.com/time/magazine/article/0,9171,952267,00.html
Saha Airlines of Iran was the last airline to keep 707s in scheduled passenger service until April 2013.

All flights of Iranian airline SAHA have been stopped due to its outdated aircrafts. The airline, which has been established by the Air Force of Iran, has operated for 23 years, but the Civil Aviation Organization of Iran stopped its operation due to outdated aircrafts. SAHA owns four planes, including three Boeing 707s with an average age of 36.8 years.

Source: http://www.payvand.com/news/13/apr/1165.html

Only the earlier 707 versions used turbojets, the others had turbofans. Saha was using the 707-320C in 2013 and still does for cargo flights. For noise and efficiency reasons the turbojet versions were probably replaced by turbofan ones some time before the 707 was removed from service.

The final turbojet-powered aircraft to carry passengers was an Aerotours Dominicano Boeing 707-020 (early name for the Boeing 720). Registration N8711E. Retired sometime between 1986 and 1996.
The final turbojet-powered aircraft to carry cargo was an Airborne Express Caravelle 6R. Registration N902MW. Retired in 2002.
Hope this answers your question!

Welcome to Aviation.SE! Do you have any sources for these details?

@Koyovis: Which is relevant how, exactly?

Would an aircraft flying for a Dominican Republic airline be registered with an "N" number, or would it begin with something else?

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What is the main difference between an ETOPS certified and non-ETOPS certified plane?
ETOPS stands for Extended-range Twin-engine Operational Performance Standards, a rule which permits twin engine aircraft to fly routes which, at some point, are more than 60 minutes flying time away from the nearest airport suitable for emergency landing.
However, it does not mean that a non-ETOPS plane physically cannot fly an ETOPS route, as American Airlines proved when they accidentally used a non-ETOPS certified plane to operate ETOPS-required LAX-HNL route. Everything went well except the cancelled return flight.
So, what is the difference between an ETOPS certified plane and a non-ETOPS certified plane when they are the same model?

I believe the constraint is related to how far they can fly with an engine out, not how far they can fly when everything works... :)

I get that, and I think OP does too, @falstro, the question is what's the difference between an  ETOPS-certified A321 and a non-ETOPS-certified A321? (or maybe that's what the smilie was for...)

@FreeMan Yeah I get that. The wording of the second paragraph might lead someone to think that it has something to do with the range of a twin. As if there was surprise that a twin could physically fly the distance, since AAL proved it by accident (if they indeed proved it by accident, I'd like to know how the f--- they do their fuel calculations...) :) Oh hey, that wording was yours btw. Didn't see that.

I am don't know the answer but one thing I heard mentioned before is the need for special maintenance procedures and specially trained engineering staff. Maybe AA saves some money by maintaining only some aircrafts to this standard? Basically it would be the same plane but some things like the APU weren't checked as thoroughly as frequently as on ETOPS-certified planes.

The number of statutory requirements they satisfy - extra safety details (analogous to insurance).

The FAA ETOPS certification requires certain standards from both the airline (the operator) and the aircraft (the manufacturer).
As far as the manufacturers are concerned, the ETOPS certification is given for a particular aircraft-engine combination satisfying certain requirements. The FAA ETOPS Rules Section A deals with the Airplane-Engine Type Design Approval. According to the document, the type approval involves a two step process:

First, the FAA determines that airplane systems meet certain design standards for safe operations during an airplane diversion.

Some of these requirements are:

The aircraft should have at least three independent electrical generators.
The auxiliary power unit (APU) (when required) can start after the airplane has been cold-soaked for several hours and can run reliably for the remainder of the flight.
System safety analyses have to show that expected system failures will not prevent safe landing at a diversion airport.
Systems with time-limited capabilities (typically cargo fire suppression) need to have the capacity to support a maximum-length diversion,
including a 15-minute allowance for a hold or go-around at the diversion
airport

The second part of the approval process is an evaluation of engine in-
  flight shutdowns and other significant airplane system failures that have
  occurred while the airplane-engine combination has been in service.

The aircraft-engine combination should have at least 250,000 engine-hours for evaluation, though this can be reduced with compensating factors. The airplane-engine combination should maintain a target IFSD (In Flight Shut Down, defined as "When an engine ceases to function (when the airplane is airborne) and is shut down, however briefly, whether self induced, flightcrew initiated or caused by an external influence.") rate at or below 0.02 per 1,000 engine-hours for 180min ETOPS approval.
The manufacturer should also demonstrate that flying with OEI doesn't cause undue stresses to the aircrew and is safe for the airframe.
The ETOPS design approval has to be applied for with the FAA and obtained after the relevant processes. Thus, the same aircraft will not have ETOPS approval if the engine is different or even the same aircraft-engine combination won't have ETOPS approval if used by another airline or if no certification was sought.
There is another method of getting ETOPS approval called the 'early' ETOPS approval which

...takes a systems approach to the development of an airplane and engine. ... an applicant must demonstrate that the design flaws on previously designed airplanes are not present in the new airplane. The applicant must also consider how the maximum length flight and diversion affect the design and function of airplane systems to ensure that they have the capability and reliability for safe ETOPS flight.

After being ETOPS certified, the airplane-engine combination and the operator are under continuous monitoring with the certificate being withdrawn in case of lapses or if the IFSD rate goes above the target rate and the manufacturer's corrective action does not eliminate the unsafe condition.
As far as the LAX-Hawaii incident is concerned, according to American Airlines Spokesman, both aircraft (the 'correct' ETOPS airplane, A321H and the wrong one sent, A321S) had the same configuration, but

There are two small differences in terms of equipment to allow ETOPS certification. Since you can’t divert for medical [on a Hawaii bound flight], you’re required to carry extra medical oxygen on board and an extra fire suppression canister

There are also reports that the (ETOPS certified) A321H has extra fuel tanks, which the (non ETOPS certified) A321S didn't have, though the AA spokesperson had said that the fuel tanks and range were the same.

So what's the difference between an ETOPS certified A321 and a non-ETOPS certified A321 of the same airline?

@Simon From Guardian "There are two small differences in terms of equipment to allow Etops certification. Since you can’t divert for medical [on a Hawaii bound flight], you’re required to carry extra medical oxygen on board and an extra fire suppression canister". Also the (ETOPS certified)  321H is supposed to have extra fuel tanks, which the (non ETOPS certified) 321H didn't have.

@aeroalias This ought to be added to the answer as this is really what the question is about, and not so much type certification or ETOPS generally.

@Relaxed Edited and added. I've left the original answer as is though.

(+1) Thanks! While we are at it, do you happen to know anything about some extra maintenance requirements (see my comment to the question) or is it something I misremembered?

IFSD = In Flight Shut Down?  If so, may want to add that in.

@Relaxed The FAA ETOPS Rules (which I've linked) Section B lists some extra maintenance requirements, like different MEL (exrta generator etc.), engine condition monitoring (oil consumption etc.) and in-flight APU restart program.

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text:
Are there any known incidents of ejections by capsule? What are the survival chances?
One hears of many stories of ejection seats but rarely of ejection by capsule. Probably because seats are so much more common.
So are there any known incidents of ejections by capsule? F-111 was one such aircraft that had an ejection capsule I recall.

Sad B-1A accident while flight testing in 1984. Rockwell test pilot died in the capsule from injuries sustained on impact

While the F-111 had a custom build escape capsule integrated with the cockpit, escape capsule designs have been used in the past.  One notable type was the Stanley Corp. Ejection Capsule.  It was a special type of ejection seat which used a pressurized clamshell-type enclosure which could be deployed rapidly once the ejection process had been initiated.  It was designed specifically to protect a pilot against a supersonic, high altitude ejection which would have almost certainly been fatal using a conventional ejection seat.  The system was used on both the B-58 and XB-70 aircraft.

The system was pressurized allowing a flight crew to use standard life support equipment as opposed to a full body pressure suit for high altitude flight.
One notable ejection involving the Stanley capsules was the loss of XB-70 prototype No 2 on 8 June, 1966 near Barstow, CA.  During a formation flight of several other aircraft with the XB-70 for a General Electric photo shoot, an NF-104 collided with the starboard wing and tailfin of the XB-70 and disintegrated in flames, killing NASA test pilot Joe Walker in the process.  The Valkyrie continued on straight and level, then waltzed out of control and began tumbling toward the desert, spilling plumes of fuel vapor as it went.  Commander Al White initiated the ejection process using his Stanley capsule and egressed the aircraft but was seriously injured by failing to clear his arm from the clamshell enclosure during deployment.  The copilot, Carl Cross, failed to initiate ejection and rode the crippled Valkyrie to the desert floor, killing him in the process.

And it was used with mixed results when the 2nd prototype of the XB-70 crashed in 1966. The XB-70 capsule was not identical, but was similar in concept, to the B-58 capsule.

Carlo, did you watch the Valkyrie video you posted? It does not show the collision or ejection, it is just 20 minutes of formation flying...

The new video is a little better.  Though the MAC and subsequent ejections were not filmed (or never released to the public), this movie contains stills of the accident.

The F-111 capsule was great and I always felt glad to have it, especially at high speed. However in my 8 years flying in it, I'm not sure that the ejections I knew about were much different statistically. The fatal outcomes were late, upside down in a rapid roll or otherwise outside the parameters and the successful ones were mostly okay with a few non-grounding back injuries caused by the impact.  One case in Mountain Home, the Wing CC pilot broke his back after ejecting a few seconds after takeoff. He was paralyzed but the WSO was not seriously hurt. My impression is that these results are probably comparable to seat ejections.

Welcome to the site!

From Sep 1966 to June 1971, I was the Avionics Officer on the F-111 Joint Test Force (JTF). I was assigned to the Tactical Air Command's Det 1 of the 831st Air Div...we, along with some AF System Command personnel, supported the F-111 JTF at EAFB.  In the late '60s (1968 or so), our Det CO (Col Henry W. Brown) and Det Ops Off (Lt Col Joe Jordan) were flying a gun fire mission and the aircraft's bomb bay caught fire (the M-61 gun was located in the right hand side of the bomb bay) and that led to the first F-111A capsule ejection by an all USAF crew.  Both crew members survived...in fact, they were sitting on a small boulder in the Mojave Desert smoking cigarettes while waiting for rescue vehicles to show up.  I recall only Lt Col Jordan was injured...he had back problems due to the "pop gun" effect of the ejection.  This incident was subsequently covered in the USAF Safety Magazine a few months later.
P.S. During the gun fire mission I was in the Ops Office and heard the entire comm between pilots and the JTF engineers and pilots.

Yes, in the B-70 accident, one of the crew members escaped inside a capsule, but had part of one of his elbows shaved, as it was caught by the shell and cut on launching from airplane, and some backbone fractures on landing, as the parachute didn't work properly, speed when capsule hit ground was too high.

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Do frequency response / Bode plots for an aircraft or helicopter come from modeling or empirical testing?
I was intrigued while reading an article by this snippet:

Examination of the V-22’s frequency response plots (Bode plots) in
  2000 showed that the aircraft failed to meet the basic stability
  requirements mandated for rotorcraft and was indeed PIO-prone.
  Since 2000, the Navy and Bell/Boeing have stated that the frequency
  response of the aircraft has been modified to remove this tendency.
  Updated Bode plots have not been made available to date.

My question: How does one go about obtaining frequency response plots for an aircraft? Is it a modelling exercise or an  empirical one?
http://www.g2mil.com/V-22safety.htm

Practical example at Nasa: In-Flight Stability Analysis of the X-48B Aircraft

@mins So, it was generated on the fly?

@sanchises: Yes, the excitation inputs are generated on the flying aircraft. The aircraft is remotely piloted from the ground station (flight test pilot and engineer). The analysis is done in quasi real time on the ground, using a set of measures downloaded from the aircraft, a new set of loop excitation commands is uploaded, etc. The stability margins are computed using a RTSM software.

@mins Yeah, I was just going for the pun there, sorry. Thanks for the link though, because coincidentally it describes something quite relevant to a project I'm doing.

@sanchises: Shame on me! Yet a good one.

The answer is both modelling and/or empirical methods are used.
For aircraft with a closed loop Flight Control System then the clearance requirements before flight requires that sufficient gain and phase margins exist for the system. For example with Military Aircraft this was defined in MIL-F-9490D. This would involve building mathematical models of the aircraft and control laws and calculating margins from the frequency response of the model. This allows the engineers to analyse the robustness of the system under various tolerance and failure states before flight.
It is possible to analyse stability margins from flight data, but note even the RTSM software referenced in the comments talks about comparing the flight data calculated margin against an a priori model.
Also note that Handling Qualities requirements, i.e. saying it is PIO prone, are additional requirements over and above the basic stability requirements. This introduces another feedback path with a pilot in the loop. For this there are additional frequency response parameters to meet, for example average phase rate. Again examples in MIL-Spec requirements MIL-F-8785C, these will all be assessed using mathematical models.


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How does yaw relate to pitch?
I have been confused about the relation of yaw and pitch in propeller powered aircraft. When such an aircraft is pitching up, will it yaw to the left or right? Conversely, when it's pitching down, which way will it yaw?
When I try this out on X-plane, the plane turns left when I take off. However, some online material insists that the plane will yaw to the right when I am pitching up. This is confusing me. What's the best explanation for this?

You need to know the direction of rotation of the propeller. There is no right or wrong answer when that detail is unknown. Look for answers in a search for p-factor here for an explanation.

How does yaw relate to pitch?
There are four forces at play on a single engine aircraft that can cause turning tenancies or Yaw.
The main reason a single-engine propeller aircraft yaws left at T/O is due to  Asymmetrical Thrust (P-Factor). At high angles of attack the right side of the propeller disc creates more thrust than the left side and this causes Yaw to the left.
Gyroscopic Precession causes a yaw to the right when an aircraft is suddenly pitched up(tail is lowered or nose is raised) and a yaw to the left when an aircraft is suddenly pitched down. (tail is raised or nose is lowered).
This force is only apparent while the aircraft's Pitch is changing, once the pitching action is stopped, the force is gone. (not present in a steady nose up climb)
Slipstream effect causes a slight yaw to the left, and slight roll to the right.
Engine torque causes a roll to the left.
Most single engine aircraft today have a clockwise spinning propeller. All these forces would be the opposite if it were a counter-clockwise spinning propeller.
A more detailed explanation can be found here: Turning Tendencies

This answer has all the right information in it, but I feel that it could be improved by highlighting those forces that are directly related to pitch change, as the OP asks.

pitched up (tail is raised or nose is lowered) ... pitched down (tail is lowered or nose is raised) I'm not sure your parenthetical statements match the ones before them. Did you get those reversed or am I misunderstanding something?

Yes, I got it reversed. I have corrected it now. Thanks. I have also highlighted the sections pertaining to pitch and yaw.

I had been reading up about the effects caused by forces at play but did not realize that these are different forces. I had gyroscopic precession and p-factor mixed up - this cleared everything for me. Specially fig 17-45. Thank you very much!

From personal experience when pitching up on climb out after take off, I have to apply right rudder because the aircraft wants to yaw left. I've been told its because of engine torque at high power and slow speed.

Why did this answer get a down vote? It looks okay to me, and if there is something wrong with it, I would love to know why.

The main reason a single-engine propeller aircraft yaws left at T/O is due to  Asymmetrical Thrust (P-Factor). Engine torque causes a roll to the left, but not yaw.

I downvoted because this answer is incorrect. The question asks about turning tendencies related to pitch. The correct answer will include two factors: gyroscopic precession, and P-factor—(though the later is more accurately related to angle of attack, independent of pitch). Torque does cause a turning tendency, but it is entirely independent of pitch or pitch changes.

Thanks for your answer. Mike's response addressed the specific question I had. Cheers!


text:
What is the relation between airspeed and rate of turn?
I am reading about aircraft maneuvering and the end of section check contains this question:

During a turn, if you reduce the airspeed and maintain the angle of bank, the result is a _____________ rate of turn.

The correct answer is "higher" which is confusing me.

If I reduce the airspeed, won't it take longer to complete the turn, thus "lower" rate of turn?
I can understand that the turn might be smoother and more coordinated if I reduce the airspeed, but how can the rate of turn be higher?
What am I missing here?

Look here for the physics behind it. With speed the turn radius increases, so it takes longer to fly the circle. Rate of turn is inverse to speed.

Independent of airspeed, angle of bank gives you the "G" force in a level coordinated turn. A 60 degree bank = 2 G's, at any speed.
At low speed, a given G force will give you a greater heading change than it would at a high speed. Driving your car at 10 mph you can complete a U-turn, 180 degrees of heading change, in a couple seconds without pinning passengers against the door, but at highway speeds, even a hard turn that does impose big side-loads will take much longer to give you that much heading change.
Related to all this is turn radius: at low speed, your turn radius for any given loading (bank angle) is smaller than it would be at a higher speed with that same loading/bank angle. Thus, even though you're going slower, you have less distance (circumference of the circle) to cover, and so you are able to complete the circle (or one 360th of it) more quickly.
((For the pedantic ones, everything here assumes level, coordinated turns. Change those assumptions enough, things can be different, but those aren't particularly instructive cases until the base case is understood.))

Ah got it. Thanks for the examples - really put things in perspective!

Excellent analogy. Never thought to explain it like that before.

The rate of turn is inversely proportional to the (True) airspeed. For an aircraft in a level, coordinated turn, the rate of turn is given by
$\mathrm{Rate\ of\ turn} = \frac{1091 \tan\theta}{V}$
where

Rate of turn is in degrees per second,
$\theta$ is the bank angle in degrees, and
$V$ is the TAS in knots.

So, as the airspeed decreases, the rate of turn increases and vice versa—as long as the angle of bank is kept constant.

Image from cfinotebook.net
For an aircraft in a coordinated turn (i.e., no skidding or slipping), the vertical component of lift is equal to the weight, while the horizontal component is equal to the centrifugal force.

Forces in a turn, image from Instrument Flying Handbook
As the aircraft turns, if the airspeed increases with the bank angle held constant, the radius of turn increases with the square of the speed ($r = \frac{V^{2}}{11.26 \tan\theta}\ \mathrm{ft}$). Hence, the distance traveled during the turn increases as the square of the speed. Even though the aircraft is flying faster, the distance to be flown increases faster than the speed. As a result, the time taken to complete the turn is increased and the turn rate decreases.

The cfinotebook image was super-helpful. Thank you for the effort! Marking Ralph's answer as the car and highway u-turn makes it very intuitive to remember this.

Rate of turn means how quickly the plane's nose is changing direction, or how tight a circle its making, turn radius, like a car.  You can make a sharper turn at slower speed.
The airspeed will be slower as you travel through the air, but you'll change direction faster. This also creates G-forces than can damage the wings etc. so banking too much at high speed, you'll feel just like a car going to fast around a turn, and you'll naturally slow down a little.

"banking too much at high speed, you'll feel just like a car going to fast around a turn" Actually, you won't. In a car that is turning, centripedal forces will force the occupants to the outside of the turn. In an airplane (I don't know about helicopters), you normally don't just bank to turn, but also use the rudder to fly a coordinated turn. In a coordinated turn, the gravity vector remains perpendicular to the wings throughout the turn, so no sideways force is experienced by the occupants. Wing loading is an issue that needs consideration, but that is unrelated to turn coordination.


text:
Is the separation distance between two aircraft approaching an airport the same under CDA and Non-CDA scenario?
I'm working on a project that investigates Continuous Descent Approach (CDA), the procedure by which an aircraft approaches an airport for landing by descending continuously from cruise altitude with idle or near-idle thrust setting. As a Continuous Descent Operation (CDO), CDA provides operational benefits in terms of fuel burn reduction which reduces emissions as well as noise levels around airports.
In my project, I'm studying CDA operations during high traffic periods with the aim of developing key performance indicators (KPIs) for airports. However, I really need some expert and professional advice from pilots and air traffic controllers (ATC) on how CDA is being coordinated, managed, and conducted between pilots and ATCs. The following questions underscore my point:

If a pair of jet aircraft is approaching an airport, is the separation distance between them smaller, the same, or larger under CDA and Non-CDA scenarios?
Just before touchdown on runway, is the aircraft's approach speed the same under CDA and Non-CDA scenarios?
In general, how many track miles could be used for approaching traffic at a major airport?
In general, is the airspeed during descent different between CDA and Non-CDA scenarios?
Just before touchdown, will the pilot utilize thrust to increase speed or will the aircraft be on its own volition, under CDA and Non-CDA scenarios?

Discussions really don't fit the format of this site, have you tried contacting controllers directly? Most control facilities are happy to arrange a visit where you could pose these questions to controllers directly.

While discussions aren't a good fit here, I think the questions should be answerable. Perhaps they should be split up at least into ATC versus pilot subjects.

Thank you for your reply, Ron. I honestly didn't try to contact ATCs, but I will grateful if you suggest a way in which I can reach out for them, or even better, visit an air traffic control facility.

We do have a question with some tips for getting ATC tours, at least for the US.

Continuous descent operations do not warrant a change in separation minima. The same separation minima (wake turbulence, radar minima, lateral, horizontal and vertical minima) apply at CDA airports as they do at all other airports.
To answer 2, 4 and 5 at the same time: continuous descent approach means that an aicraft descending from cruise level (typically 10-11 km.) to the final approach procedure (typically about 1000 metres) will do so in one continuous motion - e.g. without levelling out and maintaining a certain intermediate level along the way.
Although it is possible, and widely utilised, crews are not required to use idle engine thrust for the descend. It may well be the case that an aircraft is required by air traffic control to maintain a certain speed during descend, in order to establish and maintain the separation requirements mentioned above. As such, you can see CDA operations where aircraft apply thrust during descend, but where the descend is still conducted as one continuous motion.
On the final approach, usually starting at an altitude of about 1000 metres, 20 or-so km. from the airport, there is no difference between CDA and non-CDA operations. Aircraft will fly completely normally, using engine thrust and lift augmentation devices (flaps, slats) to maintain an appropriate airspeed (actually, angle of attack) so that the aircraft descends at the correct rate to hit the runway. For a normal jet aircraft, it is not possible to fly the final 20-or-so km. of the approach with idle thrust - it simply wouldn't make it to the runway.
When talking about track miles, I will assume you are talking about the distane from the top of descend (TOD) until the runway threshold (THR). For an average jet airlines, descending from a level of 10-11 km., it will take somewhere between 100 and 120 nautical miles of distance to get from TOD to runway THR. For non-CDA airports, this distance will generally be longer, because the descend has to be initiated sooner, but you will also find examples where there is almost no difference.

Thank you so much, J. Hougaard, for this detailed information! I really appreciate it!

If a pair of jet aircraft is approaching an airport, does the separation distance between them is the same or larger, under CDA and Non-CDA scenario?
Ans: Same. U.S. separation starts are in FAA Order 7110.65. It has one set of standards
Just before touchdown on runway, does the aircraft approach speed is the same, under CDA and Non-CDA scenario?
Ans: Generally the same.
In general, how much track miles could be used for approaching traffic at a major airport?
Ans: Many instrument approaches are defined for 15 NM (the Initial Approach Fix). However, often aircraft are cleared to the Intermediate Fix, about 10 NM from the runway.
In general, is the airspeed during descent differs CDA and Non-CDA scenario?
Ans: I don't know.
Just before touchdown on runway, will the pilot utilize thrust to increase speed or aircraft will be on its own volition, under CDA and Non-CDA scenario?
Ans: Just before touchdown, the pilot will generally initiate a flare maneuver to reduce the impact.

authors: Благовест Златев


text:
What are some of the longest flights without modern navigation systems?
Context: As a private pilot, I enjoy preparing my flights with a mix of paper charts and iPad apps. In flight, my navigation is now often very assisted by the G1000 or iPad, relegating the paper maps to backup. But I remember that, when I was only using paper maps during training, the workload could quickly rise and that I was sometimes exhausted after a short navigation because of that.

What are the longest (either by time or distance) known flights ever flown without any "modern" navigation system ?
I'm interested by flights that comply to the following criteria:

Did not use any GPS or inertial navigation system (maps and stars are ok)
Radio navigation is ok (but bonus point if the aircraft is not equipped for that)
Have only one leg
Take off and landing fields can be the same if the flight was long enough to require actual navigation (recon missions, circumnavigation...)
Can be historical stories (war heroes, military experiment, pioneers...) as well as modern ones (record attempts...)
May have a flight crew as large as Pilot + Co-Pilot + Navigator + Engineer, but...
...don't have any rest period for the crew (no backup crew on board)

Would Charles Lindbergh's flight across the Atlantic count?  1927, no GPS, only one leg, no rest period.

Yes of course, this is exactly the kind of stories I'm looking for. Maybe there are some other that are worth looking at out there :)

Lindbergh crossed the southern tip of Ireland only about 2-3 miles off track on pure dead reckoning.  That was after flying a great circle route that had several heading changes, hallucinating during the overnight phase, and eventually repeatedly falling asleep and catching himself drifting into a spiral this way or that then estimating a correction.  At the start of the second night when he arrived over Paris he realized he had enough fuel to continue to Rome, and mulled it over before deciding that would be pushing it too much.

If not distance but duration is important, certainly airship flights should lead the list.

This list of flight endurance record on Wikipedia lists as number 3 an endurance record of 84 hours 32 minutes between May 25th and 28th 1931. It seems reasonable to assume that no modern navigation means were used in 1931.
However, the flight was mostly local, so navigation was not really required.
The all time record is from Dick Rutan and Jeanna Yeager in 1986. The Rutan Voyager had state-of-the-art navigation on board, including an Omega Navigation System and a GPS receiver. The latter was only functioning for approximately 4 hours per day, because the GPS constellation was not yet complete and many satellites were missing.
The Double Sunrise flights seem to qualify, with times in the air between 27 and 33 hours.

Most of the flights on the wikipedia list seems to be "local" and required little to no actual "navigation". But the 1986 story sounds interesting. I'll try to figure out if they had a GPS.

@QuentinHayot They had GPS, but it was only working 4 hours per day. And it wasn't anything like a G1000 or an iPad

Just saw your edit about the navigation systems on board. Still an impressive performance!

There is a good point in that if you're only trying to stay in the air as long as possible, you just fly in circles around the airport. No issues with airspace clearance or traffic, no navigation necessary, easy communication with ground support, you can land at the last possible moment, you start somewhere where you know there's going to be good weather...

Don't Forget the flights over the poles:
May 9, 1926 - Richard Byrd and Floyd Bennett in Fokker tri-motor made first flight over North Pole from Spitsbergen Island and back, a distance of 1,535 miles miles in 15 hours, 44 minutes. There is some question whether they reached the pole because the flight took 3 hours less than expected for the distance.
Nov. 28-29, 1929 — Comdr. Richard E. Byrd, Bernt Balchen, Harold June, and Capt. Ashley McKinley in Ford tri-motor monoplane Floyd Bennett make first flight over South Pole, 1,600 miles, from Little America over pole and back, in 18 hours, 59 minutes.
Just a month after the Lindbergh flight:
June 28, 1927: The first non-stop flight between the United States and Hawaii is made by U.S. Lts. Albert F. Hegenberger and Lester J. Maitland. They fly 2,407 miles (3,874 km) from Oakland to Honolulu in 25 hours, 30 minutes.
Although shorter and not solo, this flight was noteworthy for several reasons:
They flew east to west, which meant flying against the prevailing winds.
Their target were islands in the middle of the Pacific - not a huge landmass.
This opened up the Pacific for commercial aviation.

and years later it put hegenberger's name on the approach road from Hiway 17 in oakland to the oakland airport terminal.

While a slightly shorter distance than the Rutan Voyager, in 1949 the Lucky Lady II did the first nonstop circumnavigation (using air-to-air refueling).  Flight time was 94 hours 1 minute, and a distance of 23,452 mi (37,742 km).  Looks like it predates any airplane use of Inertial navigation systems.
Unfortunately this flight had multiple pilots on board, and so fails the "backup crew" rule, but I think is still worth mentioning.

I love this! Even with the backup crew it must have been a pretty hard flight! The picture of refueling on the Wikipedia page is insane.

The first thing that came to my mind was the Pan Am Clipper flights, especially those operating about the Pacific / China.
https://www.clipperflyingboats.com/transpacific-airline-service


text:
Guidelines for using a constant speed/adjustable pitch propeller
I have been trained in a single engine fixed pitch prop ( C152). I'm trying to move to Piper Arrow II which has an adjustable pitch propeller. I understand how constant speed propellers work. I'm trying to understand how to use this new feature in cruise flight.
My understanding is that in the highest RPM configuration, the propeller will provide the highest thrust. This implies that the lower RPM will result in lower thrust ( true? ). If the objective is to get to the destination in the shortest possible time ( the most common objective ) why would anyone want to mess with the pitch of the propeller?
I understand that using a lower RPM may have other advantages like lower engine temperature, lower noise and possibly longer engine life. I'm hoping there is an answer like: ( you can reduce the pitch but maintain cruise speed ..) :) OR for a significant reduction in noise level you will sacrifice 5kts in speed.
This answer doesn't answer my question.

Until now, more throttle means more RPM and more thrust for you. With a constant speed propeller, you need to see the effect of the controls differently: Now the throttle controls only power and the prop pitch controls RPM. Since power and RPM are still related, make sure you have enough RPM so the propeller can absorb the desired power. If you reduce power, reduce RPM a bit as well. That's all. For the figures (how many kts per 100 RPM reduction) consult the manual.

why would anyone want to mess with the pitch of the propeller ?

A propeller runs at its highest efficiency when used at a particular combination of forward speed and RPM. By changing pitch, you change this combination, so the best efficiency can be shifted over a range of airspeeds.
For the physical background please consult this answer.

This implies that the lower RPM will result in lower thrust ( true? )

Yes, lower than maximum thrust. If you need less than maximum thrust, a lower RPM is sensible to reduce engine wear. Reduce RPM only if the remaining prop speed can still absorb the power; in other words, make sure the RPM is high enough to allow the engine to run at the speed needed for the desired power. But do not control thrust with RPM - this is what the throttle is for.
A piston engine outputs a constant amount of power, regardless of airspeed. This power output grows about linearly with engine RPM. You control this power with the throttle (and measure the power indirectly with the manifold pressure) and use the prop pitch to allow the engine to run fast enough to create the desired power at all. If you run the prop faster by reducing pitch, it will operate at a blade lift coefficient which is lower than that of optimum blade L/D, so you lose efficiency. Also, the engine will run faster and be in the partial-load operational range where it also runs less efficiently. Run the prop slower by increasing pitch and you will slow down the engine so it cannot produce the desired power. Also, now the propeller will run at a higher lift coefficient, efficiency will drop and part of the prop blade might even stall if you increase pitch too much.
As to what is a significant reduction in noise: This is subjective and I advise you to find out for yourself.

I'm confused by your answer. If I understand your other reference answer correctly, an increase  in efficiency of the propeller should result in increase in thrust. But you also confirm that the increase in blade angle which results in an increase in efficiency will lower the RPM and thus reduce thrust. So is it just a matter of playing with the throttle control to find the sweet spot where the thrust as measured on the airspeed indicator will be maximized?

@PrashantSaraswat: It's not that simple. The engine outputs a certain amount of power at a specific RPM, and you adjust blade angle to convert that power to thrust in the most efficient way. If you increase blade angle more, some of the blade will stall and thrust will drop again, even at the same RPM. The sweet spot you want to find is between RPM and power (measured as manifold pressure), where RPM must be enough to allow the engine to produce this power but not more to keep the propeller at its best efficiency.

Thrust of the propeller is a function of propeller RPM, and of the blade angle of attack:

With a constant blade angle of attack, thrust increases with RPM.
When the airplane picks up speed, the inflow angle into the blade changes: local AoA reduces, and thrust reduces. This can be compensated for by either increasing RPM or by increasing blade pitch, which is the better option.

At airspeed zero, angle of attack is the same as the blade pitch. You can adjust the pitch of the propeller to keep the local angle of attack the same when the plane picks up speed.
You get the lowest specific fuel consumption at the highest L/D of the blade, usually at a blade AoA of about 6°.

Understood, so you are saying that for a higher prop pitch angle, you will reduce fuel consumption without sacrificing airspeed and might actually gain airspeed ( as thrust will increase ). Unfortunately, in the FSX with CLS piper arrow II I didn't see any change in airspeed. Maybe they just didn't design the plane properly :). I will try out in an actual airplane.

What a constant speed propeller really offers is the ability to operate at the maximum theoretical effiency of the propeller for a desired engine speed throughout the airplane’s entire flight envelope.  Propeller efficiency is defined as ratio of thrust horsepower available to propel the aircraft forward over the brake horsepower delivered to the propeller by the engine crankshaft.
Gasoline engines produce specific power outputs at specific engine speeds.  Maximum power output orrurs at the some particular engine speed, generally redline (maximum engine speed).  Since a fixed pitch propeller cannot alter the propeller pitch, it has to be designed for certain flight regimes where it provides the best propeller efficiency.  This generally doesn’t not occur on the ground at zero airspeed in order to prevent engine overspeed as the aircraft gains airspeed and allow for a larger operating envelope at full throttle.  Airplanes fitted with cruise propellers having higher blade angles of attack and consequently lower max engine RPM during the takeoff roll,  making for longer ground rolls and sluggish climbs to trade off for faster cruising speeds.  Planes fitted with climb propeller accelerate and climb better at lower airspeeds but can’t cruise as fast without engine overspeed risks.
Since a constant speed propeller can alter the pitch of the blades to adjust engine speed in flight, it can then increase or decrease the workload applied to the engine crankshaft, maintaining a desired engine speed and power output for said speed which is then converted into thrust with minimum drag penalty.
Engine power output settings are then a specific combination of manifold pressure and engine speed.  Manufacturers publish these for specific engines and propeller combinations in the form of power charts.  These guidelines should be observed to obtain the required power outputs and fuel burn during all regeimes of flight.  An example power chart is below from a Piper PA-28RT-201T Turbo Arrow IV.

Example:  Suppose you wanted to cruise at 65% power at 12,000ft pressure altitude, assuming standard conditions.  Using the power chart 28.2” and 2575 RPM will be required for the desired power output at the lowest manifold pressure.

You can think of a constant speed propeller similar to the transmission on a car. Once at cruise altitude, you start dialing in more propeller pitch (seen on RPMs), and lowering the throttle (seen as Manifold pressure) which has similar effect to shifting up gears in a car.  If you want to climb more, you have to undo that, similar to downshifting in a car. Manifold pressure drops with altitude, so at some point you can't reduce the throttle anymore as you'll have it full in.  Don't forget to lean the mixture, generally above 3,000', so you're not running too rich and just wasting gas. Then enrichen on the way back down.
The advantage is more speed at a lower power setting, and lower fuel consumption.
Arrow II has a 360 cubin inch, 180 HP engine, similar to my Cardinal, except yours will be fuel injected (IO-360, vs O-360).
You can run flat out if you want - the propeller governor should limit the RPMs to 2700, you are running over redline if you go higher, bad for the engine. There is a safety-wired screw adjustment on mine to set the max RPM allowed.
And it's also noiser. Settle in at 2400 RPM, 24" manifold pressure, enjoy the ride, you'll be burning 9-10 gallons per hour (see page 9-4, 9-5, 9-10 of the POH). You can bump both up, but you won't have the horsepower to go all that much faster due to drag, and you'll use a lot more gas doing it.
Example Pilot's Operating Manual (also known as POH)

Here is the answer I got from my piper dakota instructor:
When you are taking off and climbing you do this at max power and lowest blade angle. When you reach cruising altitude and start leveling off, the RPM starts increasing and threatens to go beyond the red line (even though the engine is rated at 2400RPM). In this situation to continue to get maximum thrust within the power envelope of the airplane, you can adjust the pitch to lower the RPM.


text:
What will happen if an X shape wings mounted on biplane?
The question is What will happen  if an X shape wings mounted on a light biplane? What will happen in the place which upper wings come closer to lower wing?

What do you mean by "X-shaped wings"? Do you mean wings that are angled away from each other, so that they are closer together near the fuselage, and farther apart away from the fuselage? And do you have a more specific question than "what will happen"? Because as far as I know, the answer is going to be something along the lines of "it will fly".

Drawing from the hypothetical of "could a starwars X-Wing fly" which is brilliantly answered by James Gull here it is possible to achieve flight in this configuration

...It's not a very aerodynamic design. All those intersections at the
  fuselage will create a ton of drag due to interfering aerodynamics.
  The engines and wingtip pylons are not helping in that regard either,
  as they are not well blended in. The wings are essentially a biplane
  configuration, which definitely works. The designer would need to take
  extra care near the fuselage due to the surfaces being so close
  together; their wakes would interfere with each other. Careful tuning
  could help turn this into somewhat of an advantage in slow flight.

The Strop SP-7 was an early proposed design for such an aircraft. But it only
remained in this configuration for takeoff and slower flight segments, the wings folded into a mono plane configuration for high speed cruise. I cant find any evidence that it ever actually took flight.
Some aerodynamic "trials" have been done on the "design".
If you are asking about an "X" looking from the top down, it appears that Sikorsky dreamed up something like that and went as far as running tests on the design as well as building some of the major components but the project was ultimately canceled and it never flew.

Seen head-on, the Rutan Quickie https://en.wikipedia.org/wiki/Rutan_Quickie could almost qualify as an X-wing.


text:
Am I correct about how airships control their altitude?
I have read about how airships work, especially about how they control their altitude. Because what I understand from readings about the Zeppelin airships is that they were able to perform long range trips, with several stop-and-go. Also, some airships were subject to several low and high flying variations.
So my question is: How is altitude controlled aboard Zeppelins?
My understanding is that the Zeppelins weights W, and is made of X ballonets that creates a S strength equals to W. All altitude manoeuvers are then performed using vertical motors and the lift created by the movement of the fuselage. At higher altitude, higher external pression allows the ballonets to expand and thus to creates enough lift, despite surrounding air being lighter. Am I correct?

Related: When an airship climbs the hydrogen in the ship's cells expand. Does this mean that when the airship descends the empty cells fill with air?

Also related: How do blimps control altitude?

A zeppelin leaves the ground by dropping ballast. It can then take a nose-up attitude and climb on buoyancy and engine power.  As it climbs, the gas in its cells expands and when they are full, climbing further causes them to go overpressure which vents gas overboard. Then, when the zeppelin descends, the gas contracts and the cells contract and, displacing less volume, produce less lift. Then the zeppelin must dump ballast to maintain altitude.  This altitude cycling consumes ballast and when all the ballast is gone, the zeppelin must come in to land.
The last of the zeppelin designs used condensers to extract the water content of the engine exhaust and store it as ballast, so as to maximize range before replentishing ballast became necessary.

I'm sure you didn't mean it: Air does not enter the cells, only the envelope, when the Zeppelin descends. Buoyancy is already lost when the overpressure lets the cells vent hydrogen (above the Prallhöhe). Also using fuel made the ship lighter, so constant adjustment was needed. Dynamic lift could help, but only a bit (at top speed maybe 20% of mass could be carried by dynamic lift), but woe betide the ship that tries to land (and stop!) when flying too heavy.

@PeterKämpf,will edit! -NN

A minor point, but surely the bouancy of hydrogen in air is independent of the pressure (and therefore altitude); one mol of hydrogen has a mass of 2g and displaces around 28g of air if both gases are at the same pressure.

Your question is about blimps, but in the text you ask about Zeppelins. Altitude control is similar, but there are distinct differences between both types.
Blimps first: Here the envelope is pressurized (just a bit) to keep it taut when flying. Actually, the pressure is detemined by the blimps top speed: Internal pressure must be higher than the total pressure at the bow so the envelope would not be pushed in. This is done by pumping air into bags, called ballonets, inside the envelope which is otherwise filled with lifting gas. The amount of lifting gas is determined by the weight of the ship and adjusted such that the ship is balanced. All remaining volume of the envelope that is not taken up by lifting gas is now filled by the ballonets.
Climbing can be done in two ways: Either by pointing the ship up and opening the throttles, or by dropping some ballast such that balance is lost and lift exceeds the remaining weight of the ship. As the ship ascends into thinner air, less pressure is needed to keep the envelope taut and the ballonets are deflated so the lifting gas can take up a larger proportion of the envelope. Pressure is adjusted such that the pressure difference between outside and the envelope stays constant. On the way down the reverse happens and now air needs to be pumped into the ballonets to keep the internal pressure up.
Since one mol of lifting gas will displace the same mol of air, lift does not change as the ship ascends (given that no temperature difference builds up between outside air and the gas in the envelope). Climbing by imbalance means that the ship will accelerate upwards since lift is continually larger than weight. At some point lifting gas has to be vented or balance is restored by seepage. At the latest this happens when the lifting gas has expanded so much that all ballonets are empty. Safety valves will then open as the airship climbs further in order to avoid excessive pressure which would rupture the envelope.
The Zeppelin NT is a blimp, despite its name. Here is one rare case where the propellers can be swiveled up- or downwards. Most blimps and all "real" Zeppelins used fixed propeller installations, so "vertical motors" are rare. They help to make the airship more responsive but altitude control is not done exclusively by them. Instead, the traditional technique of releasing either ballast or lifting gas is used to balance the ship and dynamic lift to control vertical speed, so you are right about "lift created by the movement of the fuselage". At higher altitude, lower external pressure allows the lifting gas to expand (shrinking the ballonets) and thus to create enough lift.
Now over to Zeppelins: Here the lifting gas is held in a number of gas bags which are taking up the upper part of the envelope. It in turn is held taut by the internal structure. Pressure in the gas bags is equal to static air pressure and altitude is controled much as in blimps: Either by dropping ballast (see here how USS Akron drops water to unsuccessfully prevent the lower fin to hit the ground) or by pointing its engines upwards. Raising the nose will also create dynamic lift, so both thrust and dynamic lift will make the airship climb. As the ship ascends, the gas bags expand and push air out of the envelope. Also here, the reverse happens on the way down, only in Zeppelins the air will flow into the envelope all by itself when the gas bags contract as ambient pressure rises.
Note that altitude control in airships will be unstable below a critical speed: In order to lift the nose, the elevator needs to push the tail down, creating a downforce. At low speed, this downforce is higher than the dynamic lift which can be created from the pitch attitude of the hull, so commanding a nose-up maneuver will make the airship sink. Experienced Zeppelin commanders used this effect to make precise landings: The ship would come in heavy (static lift being a bit less than weight) and just at the right height and distance from the desired stoppig point, the engines were reversed and the tail raised by the elevator. Now the elevator lift would help to stop the descent, helped by the upward-pointing thrust component of the propellers which at the same time decelerated the ship such that it came to rest just where the commander wanted. Being a bit heavy, only a small ground crew was needed to bring the ship in. Performing this maneuver, however, required an experienced crew onboard.


text:
Can an aircraft (civilian or military) land with its brakes already engaged?
Can an aircraft land with its brakes already engaged at the max landing speed of that aircraft?

Can an aircraft land with its brakes already engaged at the max
  landing speed of that aircraft?

Yes...
Im gong to assume that by brakes engaged you mean to full, preventing the wheels from spinning. First Lets consider the two different types of planes. Tail wheel planes will most likely nose over depending on how the plane is landed. A tricycle gear equipped plane will most likely grind through the main gear. Remember the nose gear does not generally have a brake (in my GA experience it never has a brake).
Although I would not advise it. It depends really what you mean by "can it land" it will touch down and the tires will simply lose traction and start skidding (eventually to a stop). The results (on a dry runway) may be similar to what we saw with JetBlue flight 292 which had its nose gear turned 90 degrees. The gear was basically ground through like it was sanded down. The tires in this case will all begin to heat up severely and eventually fail. My guess would be that the skidding will vary from side to side and cause the plane to weather vane quite a bit as if differential braking were used.
You may also damage the runway quite severely.

My mistake, its sill early over here for me my brain is still warming up....


text:
Why are fighter jets rated differently than passenger jets?
Recently I watched a video on the economics of running an airline. They mentioned that passenger planes have a life span rated in cycles. That got me thinking because as far as I know, fighter jets are rated by flight time hours.
My answer to this was that the only thing that gets pressurized is the cockpit as opposed to basically the entire fuselage. But surely that would still limit the fighters cycling? Or can they simply change the canopy because it is the softest part and absorbs all the flexion?

Fighter parts are also rated in cycles. The hour rating is based on expected cycles / expected usage. Eg, more BFM and low level practice will wear out the airframes faster. Individual a/c will wear out faster or slower than others even if they were flown the same number of hours.

The philosophy of fighter cabin pressurization is different from that of the passenger aircraft. In case of airliners, the cabin pressure decreases while the aircraft climbs upto a certain altitude, after which the value is kept constant (usually at 6000 - 8000 ft cabin altitude).
In case of combat aircraft, the principle is different. The cockpit pressure is maintained at certain value to a particular altitude (usually 8k feet); then the pressure is maintained (at 8k ft) till the aircraft reaches another altitude (~23k feet); after that a pressure differential (~5 psi) is maintained between the cockpit pressure and the ambient pressure. This reduces stress on the airframe and is safer in case of rapid decompression during combat. So, a fighter cockpit at 40k feet is actually stressed less (due to pressurization) compared to an airliner at that altitude.
There is another thing to be noted- a combat aircraft is designed for much higher loads compared to a civilian airliner; so, the structure is capable of taking more stresses. So, pressurization is not the limiting factor in case of combat aircraft.

One way of calculating the life of fighter aircraft is to apply loads on the airframe depending on the expected mission profile (like TO- cruise- loiter- cruise- landing), then calculate the theoretical life based on how long the airframe survived. Of course, this means that the life will change if the mission changes- like what happened when some aircraft were shifted from high level bombing to low level penetration roles.

Thanks for such a detailed answer, I really appreciate it.

Many aircraft are lifed according to how much G they pull.  G meters record the maximum and minimum G and how many times a particular load is reached during a sortie.  From this, a fatigue index (F)) is calculated and once the certified FI limit is reached, the airframe must undergo major repairs or is scrapped.


text:
Is it easier to take off against the wind or with it?
I was wondering if it would be better to take off into the wind because then you would need less overall acceleration to takeoff, or with the wind behind you - thereby making it easier for the plane to fly when it's in the air?

Almost always into the wind. The ground roll will be substantially decreased because you've already got a certain amount of airflow over the wings.
Taking off with a tailwind is pretty much never advisable unless it's a one-way strip or runway, or the winds are below the tailwind limitations for that aircraft type.

you could add that a good reason for using a runway one way only is runway slope.

The speeds required for planes to fly are speeds relative to the air, not relative to the ground. You are correct that taking off with the wind will require less acceleration to get to a certain speed, but that speed is relative to the ground, so you might not be at the required air speed.
Picture a runway left to right. A plane on the right side wants to take off to the left, and a wind is blowing. Consider two cases:

The wind is blowing left to right(headwind). Any rightward wind speed can be added to the plane's leftward ground speed. If the plane can take off at, say, 50 kts, if the wind is 10 kts, the plane's required ground speed is only 40 kts. The total airspeed is 50kts.
The wind is blowing right to left(tailwind)
Any leftward wind speed must be subtracted from the plane's ground speed to get airspeed. Again, trying to reach an airspeed of 50kts, a tailwind of 10 kts will require a faster ground speed. The aircraft's speed relative to the ground will be 60kts before it can take off, though the instrument airspeed will only measure 50 kts.

So, for those cases, there is a difference of 20 kts ground speed. It can take a bit more runway to get to 50 kts airspeed if your ground speed is even faster than that. Most pilots don't enjoy running out of runway.


text:
How to calculate the airspeed for maximum specific range?
Given the following parameters for a small business aircraft:

Wing surface: $S=30m^2$
Lift Drag Polar in clean configuration: $C_D = 0.022 + 0.047 {C_L}^2$
Maximum Lift Coefficient $C_{L_{max}}=1.35$
Static sea-level Thrust: $T_0 = 12KN$
Thrust variation with altitude: $\frac{T}{T_0}=\frac{\rho}{\rho_0}$
Aircraft weight: $W = 50 KN$
Thrust specific fuel consumption (constant): $C_T = 0.7 N/Nh$

I need to calculate:

the optimum lift coefficient for maximum specific range
the airspeed for maximum specific range when operating at the specified weight and sea level conditions.

For $C_{L_{opt}}$ I have found a value of 0.395 using the lift drag polar and the fact that $C_{L_{opt}}= \sqrt{\frac{1}{3}C_{D_0}\pi A e}$.
I am then assuming that the airspeed for maximum range is given by $V=\sqrt{\frac{W}{S}\frac{2}{\rho}\frac{1}{C_{L_{opt}}}}$.
In my calculations, I am using $W=50000/9.80665=5098.581 KG$ and $\rho=1.225$, which gives me a value of $V = 26.504 m/s$
However this seems not to be the correct answer. What am I missing?

Without checking where your formulas come from: why are you using W in kg? W is a force and and the correct unit is N. You are not getting 26.504m/s but 26.504 sqrt(m).

@mns Yes you are absolutely correct! I don't know for some stupid reason I was persisting on trying to convert W to a mass whereas it is indeed a force. I am getting the right answer now: V=83 m/s. Thanks! If you put your comment as an answer I will accept it

Check the units. Using $$W = 50000N$$ instead of $W=5099kg$ you get $$V=83m/s$$ (vs. $V=26.5\sqrt{m}$).


text:
What is "Takeoff Minimum: NA" for US Airports as depicted in Terminal Procedures?
This question refers to US Air Carrier procedures for determining takeoff minimums, as published in FAR 91.175, 121.651, Carrier OpSpecs, and the US Terminal Procedures Publication booklet.
As nicely answered in this question, we know that low-visibility minimums for takeoff are outlined in a carrier's opspecs, with paragraph C078 granting approval for "lower than standard" visibility requirements, if the airport runway doesn't already have published higher than standard takeoff minimums.
An airport with a published higher-than-standard minimum for one or more runways is indicated on approach charts with the upside-down triangle with the letter T inside (although this may also indicated only an obstacle departure procedure, or both an ODP and a minimum). The published minimums themselves can be found in the US Terminal Procedures Publication.
My question is about runways marked as "Takeoff Minimum: NA". What does "NA" mean? Does it mean that "lower than standard" minimums may not be used.  Or does it mean "all other runways have comments, but no comments are necessary for this runway, so lower-than-standard takeoff minimums published in opspecs are fine and dandy"? There also seems to be different "NA" indications; some with comments such as "obstacle" or "ATC", and some without any comment at all.
I cannot find an answer in the FAA's Instrument Procedures Handbook, the AIM, or elsewhere.

One example is Columbia, CA which has a simple "NA" for runway 35.

Another example is Cloverdale, CA, and RWY 32 is marked as "NA" but has the comment of "Obstacle".

A third example is Merrill Field in Anchorage, Alaska, with the comment of "NA - ATC".

On page D1 of the TPP-Abbreviations
NA.......Not Authorized
Sometimes they give the reason why it is not authorized like in your examples. Other times they don’t. In these cases, you can’t use the runway for IFR departures. (non-Part 91)
N/A.......Not Applicable
I don’t recall seeing this terminology in the departure minimums section, so if you could show me where you found it, maybe I could explain it.

Wow. My dunce moment... I was completely reading those entries as "N-slant-A", as opposed to just "NA".  You're right... that terminology ("N/A") does not exist. Thus... I cannot show you where I found it.  smh  Thanks!

Yuck - I'd hate to ignore that as a part 91 guy and fly into some granite clouds.

I’ve been thinking about that as well. Takeoff minimums don’t apply to Part 91, but does NA? I know that you can’t use an approach that says, NA Alternate Minimums Not authorized, but can you use a departure marked NA?

NA = Not Authorized, just like in an approach minimum line
The use of "NA" in takeoff minima is identical to that in approach minima -- it means "Not Authorized", or in simpler terms, "you can't use that runway for an IFR takeoff."
The brief remarks explain why that is so:

"Obstacles" or "Terrain" mean what they say -- trying to take off from that runway in IFR risks crunchy, smashy sounds.
"ATC" means that there's some other traffic flow blocking the takeoff path, or some other reason the runway is unsuitable for an IFR takeoff (such as inadequate lighting)
"Airspace" usually occurs because a takeoff from that runway would cause you to infringe on controlled or special-use airspace.
"Environmental" could be terrain, obstacles, or some other reason

Thanks! See my comment on @jscarry's answer. Both answers are perfectly acceptable, but I'll toss "accepted answer" points his way, since he has less.  (I assume that's an acceptable community norm)


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Why is this airfoil curved upwards on the bottom part of the airfoil?
Image source datawiki.net
Is this to slow down airspeed. I know that the high pressure and low pressure above and below the wing both contribute to downwash, but how does this airfoil form downwash?

What do you mean by 'slow down airspeed'?

Also good to indicate image sources. (I grabbed it from Ethan's original post.)

Downwash is simply the deflection of air due to airfoil (or wing etc.)
Symmetric airfoils don't produce downwash (or lift) at zero angle of attack.

Source: oppositelock.kinja.com
However, as the angle of attack is increased, the (symmetric) airfoil produces both.

Source: Effect of angle of attack on flow field and body forces by Paul Nathan from youtube.com
Symmetrical airfoils are used in rotary wing applications, vertical tail (and rudder) etc.

Does a conventional airfoil produce lift at a zero degree angle of attack? I mean airfoils that are curved downwards on the top and have no curve on the bottom.

@Ethan Cambered airfoils produce lift at zero angle of attack.


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How do the blades on a propeller generate thrust?
I am having trouble understanding how this type of propeller generates thrust:

It is so much different than a propeller on a Cessna and thrust would be provided differently. I am also confused of how the turbofan blades in a jet engine provide thrust:

Doesn't the turbofan push the bypass air back really fast providing thrust?  How do each of the these propellers (turbo prop and turbofan) produce thrust?

The propeller on the ATR in your linked image functions in the same basic way as one you would see on a Cessna piston aircraft. Perhaps you are noticing that the ATR prop is in a feathered position, in which the blades are rotated to be in line with the airstream. This is done automatically by the propeller pitch controller when an engine has been shut down in order to reduce drag. On smaller aircraft, like a Cessna with a "constant speed propeller", the pitch can be controlled manually (it's actually indirectly controlled but that detail is best saved for a separate discussion).

Propellers work the same way as wings.  They produce lift with the vector facing forward and we choose to call it thrust.

Okay so I know how the blades on a Cessna spin and generate lift, but I am really confused with the picture I provided of how that propeller generates thrust.

Ethan, Please provide attribution for your photos.

@Ethan, the ATR propeller does not generate thrust in that position; it is the position which is known as feathered. For takeoff the blades must be rotated back into fine pitch.

The cross section of propeller blade is an airfoil. As the propeller rotates through the air, it generates lift, which becomes thrust as the it is directed forwards rather than vertically upwards as in aircraft wing.

Source: recreationalflying.com
The relative airflow on the blade section (airfoil) is the resultant vector of two things - the forward motion of the propeller (the aircraft) and the rotational velocity of the propeller blade itself. Most of the propellers in use today are fixed speed variable pitch propellers. So, velocity experienced by the propeller blade is maximum at the tip (where the rotational speed is maximum) and minimum at the hub.

Source: americanflyers.net
So, at any RPM, different parts of the propeller are at different angles of attack. To remedy this, the airfoil sections are varied along the propeller. So the propeller is practically twisted along its length, with the root at higher angle of attack. Also, in some cases, the airfoils are changed along the length (thinner ones are near tip).

Source: www.pilotfriend.com
As already noted, the pitch of the propeller blades can be varied in flight. What the photo shows is a feathered propeller i.e. a propeller that is set parallel to the airflow. This is done usually in order to reduce drag in case of engine failure. During normal operation, the blades will not be in feathered position. For example, in the figure below, the propeller of engine 1 feathered while the next one is not.

From wikimedia commons, work by Julian Herzog
In aircraft, propeller pitch is changed with speed, with fine pitch at low speeds and strong acceleration (like takeoff) and a coarse pitch for high speed (cruise).

Source:forum.warthunder.com
In most aircraft, the propeller pitch settings are adjusted automatically by a governor according to requirements. The following figure shows the pitch variation in flight for a similar propeller.

"Propeller pitch on an ATR 72" by Olivier Cleynen - Own work. Licensed under CC BY-SA 3.0 via Wikimedia Commons.
Thrust The thrust in both turboprop and turbofan is provided by accelerating air through fans. In a turboprop, it the propeller, while it is the bypass fan in case of the turbofan.
The thrust produced, T is given by,
$T = \dot {m} * (v_{f} - v_{0})$,
where $\dot{m}$ the mass flow rate of air and $v_{f}$ and $v_{0}$ are the final and initial (unaccelerated) velocities of the air respectively. Usually case of propellers, the mass flow rate $\dot {m}$ is higher, while in case of turbofans, the variation in velocity is comparatively higher.

The picture of the turboprop is probably confusing you because you are looking at a shutdown engine.  For a multi-engine propeller airplane a desirable feature is that the propeller with auto-feather when shutdown, so that an in-flight engine failure will cause the prop to be in a minimal drag configuration.  This minimal lift, minimal drag configuration is what you posted, and this is not producing thrust.  When the engine is started and system pressure (oil, hydraulic, whatever controls the adjustable pitch prop for that particular engine) will drive the propellor pitch angle into lift-producing regimes.  Turboprops also typically feature "Beta" propellor pitch, which allows the propellor pitch to be adjustable past zero-lift into negative-lift and is used for landing rollout and (in some airplanes, perhaps not many) slowing down in-flight or steep descents.  During normal flight the turboprop propellor is producing lift just like the propeller on the cessna you are familiar with.
The N1 fan in a turbofan (the big one you see when you look at the front of the engine) is a ducted fan.  The individual blades of the N1 fan are angled to push air backwards.  This works the same way as a big box fan you can buy at walmart.  The key difference is the N1 fan rotates an order of magnitude faster than a box fan.  The bypass air is accelerated by this fan and exits through the rear of the engine to produce thrust.

I get it. So the first pic shows the engine in shut down mode to reduce lift then the blade turns while in flight.like a cessna engine


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Why is the most common hydraulic fluid so poisonous?
The usual hydraulic fluid, Skydrol, is poisonous. If it makes it to the air conditioning system, it causes fume events that may have adverse health effects up to and including at least one fatality. And it is not easy to completely prevent the possibility of it creeping into the air ducts as bleed air is also used for pressurizing the hydraulic reservoirs (at least on some aircraft).
So why was this organophosphate-based fluid selected over anything safer? Are there no safer alternatives?

Update: I should have linked the Qantas A332 at Sydney on Dec 15th 2019, hydraulic leak prompts evacuation accident, which actually inspired the question. That one is clearly hydraulic-related (while in most other fume events we never know for sure), and there is a comment down the page:

By geezer on Monday, Dec 16th 2019 14:48Z:
Nothing to do with the apu other than the duct by the packs. Hydraulic reservoirs are (gas) pressurised by either an engine bleed or from the pneumatic duct in the pack bay. Hence a tee piece. If one system loses pressurisation, say overnight, then with a couple of leaky check valves one reservoir can slowly fill another back through the hydraulic service manifold, as all three systems are serviced from one inlet using a three-way valve. Eventually one tank is overfull to the point where the fluid now travels back down the air pressurisation pipe (a small bore pipe). Another check valve stops it flowing back to the engine so it ends up dribbling into the pneumatic duct in the pack bay. We know this because we have found skydrol in these small bore pressurisation lines. I hope you get all that. I think I need a little lie down now.

It should be noted that this is not verified and may not be the cause, but A330 has APU intake on the bottom, so the APU could have also ingested the hydraulic fluid dripping from the bottom of the fuselage into the inlet.

A big reason is that it isn't very flammable compared to other hydraulic fluids.

Yes 5606 is very flammable, with a really low flash point.  Phosphate Ester is awful stuff to be around, one of the things, besides shift work, that got me out of mechanic-ing and up into the office back in the 70s.  You have to make sure you wash your hands several times before going for a wiz.

@JohnK That fluid seems to have a lot in common with hot sauce.

Skydrol was developed to provide a hydraulic fluid that does not catch fire when a fine mist of particles under pressure is released, upon a leak in the hydraulic system. Of the two evils, poisoning or burning, a case can be made to avoid the latter when considering aeroplanes.
For ground based systems like full flight simulators, hydraulic systems are on the way out and only found in older sims. These often do not use Skydrol due to the toxicity of it, but old fashioned flammable oils like Shell Tellus. And then it makes sense to have the fire drills and extinguishers in place, although a simulator building does not have potential open flame sources such as a jet engine.
There is possible interaction between the hydraulic systems, the bleed air systems and the air conditioning systems. For instance for the B737, from this site:

The hydraulic reservoirs are pressurised from the pneumatic manifold to ensure a positive flow of fluid reaches the pumps... The latest 737's (mid 2003 onwards) have had their hydraulic reservoir pressurisation system extensively modified to fix two in-service problems 1) hydraulic vapours in the flight deck caused by hydraulic fluid leaking up the reservoir pressurisation line back to the pneumatic manifold giving hydraulic fumes in the air-conditioning ...

It should be possible to develop a hydraulic fluid that is neither toxic nor flammable, but the civil aircraft industry does not change practice until proven necessary and safe - for good reason. New developments are risky, and must be proven long term first.
Market forces and accident rates have proven to be insufficient for developing a non-toxic, non-flammable hydraulic fluid in the last couple of decades. Perhaps another tragic major disaster needs to happen first?

"although a simulator building does not have potential open flame sources such as a jet engine." Nor do they have the kinetic energy of a half-million-pound airplane moving at 150+ mph that can be dissipated in undesirable ways if contact with the surface doesn't go quite as planned.

Phosphate-ester hydraulic fluids do catch fire readily if exposed to open flame, albeit not quite so enthusiastically as the older alkane-based oils.

FYI, MIL-PRF-87257 is a better fluid all around the Skydrol, from a flammability, health, corrosion, and hydraulic sealing point of view, and it what replaced 5606 in much of the DoD fleet (83282 is another option, but has a higher minimum temperature). But switching to it in commercial would be extremely expensive because phosphate-ester and petroleum fluids need different seal materials that are not at all compatible with each other. So no one will retrofit existing aircraft, and if a new aircraft came along you could share ground equipment between the two types.

Fume events are from two sources in the vast majority of cases:  Deicing fluid, and engine oil.
The Aviation Herald event linked was poisoning from (really severe) engine oil contamination (mainly the toxic TCP additive; it's the same stuff used as a lead scavenging agent for running 100LL fuel on low octane engines, and it's also the oil additive that Lycoming specifies for the H2AD piston engines for the cam spalling problem). Phosphate Ester hydraulic fluid had nothing to do with it.
The oil contamination comes from 3 sources:

A failed compressor section seal on the main engine at some forward bearing that is able to feed oil into the compressor upstream of the bleed port used for air conditioning.
A blown seal on an air cycle machine (the turbocharger-like cooling unit in an air conditioning pack) allowing ACM lube oil into the conditioned bleed flow.
An APU compressor seal failure (more or less the same as an engine oil seal failure) getting into the APU bleed and into the A/C packs.

Probably the majority of fume events are from oil traces, only enough to make smells, but which is really hard to remove, and deicing fluid (when airplanes get deiced and too much was sprayed into the engine and the engine didn't get enough of a purge run to clean the compressor out before turning on the bleeds).  Deicing fluid fume events happen a lot and are probably the most common cause of fume turn-backs.
Skydrol/Hyjet is mostly a hazard to the techs that work on the planes.  Anyone here who turns wrenches on transport airplanes will tell you that Phosphate Ester fluid is nasty, nasty stuff to get on your skin and probably the single most unpleasant aspect of the job.

damned knowledgeable answer!

Is Phosphate Ester the reason I was told in avionics class to always wash your hands BEFORE going to the restroom? I was told everything is covered in a clear, thin film that feels like...burning...

@DKNguyen yes.  See my comment on the question post. On somewhere like a rear spar, or in a landing gear bay where you have hydraulic actuators and a lot of turbulent air, if any fluid seeps out of dynamic seals of the acuator it gets distributed everywhere you have a greasy film of it on everything,

I don't believe TCP was ever used for lead scavenging in fuel (that's EtBr2's job).  Other than that, very good answer

@UnrecognizedFallingObject  Thanks   But...TCP has been around forever.  Developed for the B-36 IIRC   http://www.alcorinc.com/wp-content/uploads/2012/06/TCP_FAQ_06_12_no_bleeds.pdf

@JohnK -- yeah, not surprising that it's been around for a while as part of oil

No, as a lead scavenging agent (ALCOR) it's been around for decades and that's what it was originally developed for.  It was discovered it had anti-scuffing properties when added to oil later (creates a sacrificial oxide layer). The Lycoming TCP product is PN LW-16702 in a little clear bottle.  It was always some mystery liquid and I found out it was TCP only a few years ago.

Actually, I should have linked Qantas A332 at Sydney on Dec 15th 2019, hydraulic leak prompts evacuation. The cause of that one was clearly hydraulic fluid, and there is a comment (from Dec 16th 2019 14:48Z) talking about the line (updating the question; it's too long for comment).

Wow.  That's a pretty bad design.  They systems I'm  familiar with have no such access path for fluid.  Very interesting thanks.

The first two paragraphs are wrong. All Boeing aircraft except the 787, and all Airbus aircraft except the A380 and A350 use Air-On-Oil reservoirs. Bleed air from the engine pressurizes the fluid in the reservoir and there is no barrier between the fluid and air. A check valve is used to prevent fluid from back flowing into the bleed system, but it has happened.
Pumps on large aircraft receive their fluid from pressurized reservoirs, not accumulators, which are used to store hydraulic energy.

@DKNguyen --- regarding washing hands before going to the bathroom, why don't mechanics wear gloves? Relatively impermeable gloves are available -- e.g. https://www.camlab.co.uk/1212979-shieldskin-chem-red-neo-nitrile-300-gloves -- which have a soak through time on the order of ~500 minutes for phosphoric acid, and in my experience still permit manual operations with good dexterity (at least compared to leather cryogenic or riggers' gloves and similar). Compared to the cost of an aircraft or a mechanic, they are comparatively cheap. Why don't you wear them?

@Landak Little parts like little bolts and screws. Plus not everyone working a plane around the fluid is a mechanic. Avionics tech, for example, little wires and such. Even with those kinds of gloves it can be a pain. Also, secondary transfer from people not wearing gloves. That said, I never worked as a tech. I just know that even on freezing days the work they were doing did not allow for gloves.

Organophosphorates reinforce Acetylcholine neurotransmitter's actions, by blocking its degradation, it have two toxicities: early and late, early is due to its action on neurotransmitters, late is induced by neuron lesions, and harder to treat.
It is not difficult treating organophosphorate acute poisoning, as long as causative agent is clearly identified, and therapy started soon; in cases of pesticide poisoning, sometimes peasants know little about the features of product.
Aviation Products with a deadly record in poisoning are Methanol, absorbed through skin and breath, Harry Ricardo lost and apprentice from this, and Ethylenglycol, used in cooling liquids to prevent it freezing. Propylenglycol is not that dangerous, reasons why its use is almost unknown in this field remain a mistery to me, perhaps production costs are much higher. Salut +

This seems to address the mechanism by which the chemicals are poisonous, but the question is why aviation fluids include them rather than something safer.

It's pertinent information though, as in that the poisoning effects can be managed and are well known. Another input into the risk management matrix.


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What causes the inside glass of flight instruments to get hazy?
Sometimes on analog flight instruments (aka steam gauges) there is a haze or fogginess on the INSIDE of the glass that is NOT condensation. You can often find it on older aircraft with older instruments. But the instruments don't all develop the haze. I've spent some time trying to determine what this is and I found a few explanations, but only one of them addresses haze/fogginess inside the glass. This is one explanation I have found:

On instruments connected to pitot static system, it is the result of small air leaks which allows air to flow into the instrument and behind the glass, overtime it causes glass to get hazy on inside due to particulates in the vacuum air.

I find this to be credible but would like confirmation.
Additionally, I've noticed similar haziness/fogginess on non-pitot static instruments like Manifold Pressure, Tachometer, and others. So, there is likely another explanation for this.

This can depend on what the actual instrument is and what (if anything) the glass is coated in.

In some cases the instrument may not have a glass panel at all, it may be acrylic or any other type of similar material. You are mot likely seeing sun damage in this case.
It may be mold or fungus. Camera lenses suffer from a similar issue. If the aircraft resides in a humid environment, maybe even trapped in a damp hanger you can get build up. Since instruments get rebuilt sometimes (some more often than others) they are opened up and resealed quite a bit in their life times. This may not always happen in a perfect environment and even a small amount of fungus can quickly damage something.
It could be interior lubricants (oil) that are thrown around the instrument as it spins at high speed.
Some of the older units are not sealed, and for that matter some may even be vented to the atmosphere as is suggested here.
It could be dust drawn in through the vacuum system that the filter failed to draw out.

Vacuum systems are very common for driving gyro instruments. In a
  vacuum system, a stream of air directed against the rotor vanes turns
  the rotor at high speed. The action is similar to a water wheel. Air
  at atmospheric pressure is first drawn through a filter(s).

A lot of materials give off 'gas' (for want of a better description) and these can settle on the interior surfaces. This process may be accelerated by heat which is generated by the unit itself or by the environment the unit is installed in. Oil-based materials are prone to this including plastics. This is also what creates that new car smell.


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Was this an unusual landing sequence for an airliner?
About a month ago I encountered an unusual landing during a flight from Bangkok to Krabi airport. We were flying Boeing 737.
We took off 30 minutes late. Our flight should've taken 90 minutes but it actually took less than 70 minutes so we arrived almost on time.
The seatbelt sign turned on around the 60th minute while we were still probably in 10000 meters (I don't know the cruising altitude for sure). The whole landing took less than 10 minutes. I guess the landing gear was deployed at around 6000 meters which seemed unusually high to me. The pilot also performed several rather sharp roll maneuvers and at one point he broke his descend and even entered a rather sharp climb. All this with landing gear out. I have to say that many passengers were quite terrified at the moment. The landing itself was hard but not that much.
I have been  wondering if it was a common maneuver (to decrease speed perhaps?) and would be interested to learn more about it. Does it have something to do with the flight speedup? How is it even possible to be so much faster on such a short flight?

Passenger descriptions of altitude, speed, bank angle, and configuration are notoriously unreliable from inside the cabin, even among experienced pilots.  I'm sure your flight was normal.

If you provide date and flight number maybe we can dig up more things for you. And relating flight schedule http://aviation.stackexchange.com/questions/23899/why-do-airlines-over-estimate-over-calculate-flight-times

@abelenky may have answered your doubt. It is quite difficulf, from the cabin, to describe maneuvers, altitudes and other pilot's duties.

@eduardoguilherme I know that my observations are probably vastly inaccurate. Still, this flight was a lot different to what I am used to and judging by the reactions of copassengers I was not alone. I have no doubt that the pilot had it under control. I was just wondering if it could be due to time, specific location of the airport or some other reason. Maybe every flight to Krabi is like that I don't know :)

@Honza Brabec, is it possible to tell us the flight number, the date and the airlines, please? And also, the flight time (origin and destination airport)

@vasin1987 SL-8574 01 Feb 2016 11:05 AM Thai Lion, Bangkok, Krabi

https://www.flightradar24.com/data/flights/sl8574/#8e80428 this route seems to always depart late and arrive on time.

I guess the landing gear was deployed - how do you know?

An airliner wanting to do a steep descent might well deploy flaps and similar devices, which could sound like the landing gear being deployed.

One reason passenger reports are unreliable is spatial disorientation Since you don't have instruments in front of you and only have a small window to look out of it's easy for the brain to be fooled into thinking the plane is doing something it really isn't. For instance you talk about a sharp climb. If you go the link posted by vasin1987 and pull up the speed/ altitude graph you will see that once

descent started the aircraft never climbed. Your senses were fooling you.

There's a free iPad app called Personal Black box that tracks where you are and displays aviation charts that you can uses for situational awareness.

Why do you think the flight "should" take 90 minutes?
You really mean it was listed for 90 minutes on the schedule.
Airlines almost always pad their scheduled times so that their on-time arrivals stats are favorable, and to account for normally expected delays.
Scheduled time is also typically measured "gate-to-gate", while most passengers only observe the flight-time (takeoff-to-landing)
I see the distance is 350 miles, and would probably take a 737 about 60 minutes of actual flight time.  So I don't think your flight was speeding or otherwise behaving abnormally.


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Is an ADS-B IN able to receive signal from ADS-B OUT of the same aircraft?
Suppose I have an ADS-B IN on my aircraft and I am interested in receiving also data sent from ADS-B OUT of my own aircraft. Is it technically possible or are there any issues due to the relative position between transmitter and receiver or anything else?

Why would you want to do that? There some cases where it is technically not possible, but in general there are no fundamental technical limitations of receiving your own ADS-B signals.

On small aircraft many operators use portable ADS-B IN receivers which typically do not have the ability to sense suppression signals from their own transponder/UAT transmissions and so they will pick up their own signal.
Most installed systems have that suppression functionality so this won't happen.
The suppression is simply a wire between the two units that if the transmitter is transmitting, it goes high, which tells the receiver to ignore whatever it is receiving at the moment.
So if you want to "receive" yourself, simply snip the suppression wire if you have one...

One of the reasons to have a suppression function is to protect the sensitive receiver front end from being blown out by a high power transmission a couple of feet away, or sometimes even on a shared RX/TX antenna. If you are looking for an effective way of making your receiver as deaf as your 95 year old granny, cut the suppression wire.